1. Field of the Invention
This invention pertains to a fluid actuator and more particularly to a hydraulic actuator having a piston and a valve means wherein a force applied to the valve means is varied resulting in a frequency change of piston reciprocation.
2. Description of the Prior Art
In U.S. Pat. No. 4,143,447 to Krasnoff et al. and U.S. Pat. No. 4,192,219 to Krasnoff et al., a hydraulically operated reciprocating piston is described wherein the piston in cooperation with a cushion chamber, a liquid supply source and a valve means reciprocate the piston.
These apparatuses are useful in hydraulic rock drilling operations. Drilling hard rock calls for high energy blow at any given power level. This is especially important when drilling deep holes. Soft rock requires a lower blow energy and force level for optimum penetration at a given power level. In the above mentioned patents, in order to change the blow energy and frequency of reciprocation of the piston it is necessary to change the design of the apparatus. A single rock drill actuator of small size and high efficiency for both hard and soft rock is desirable.